1. Field of the Invention
This invention relates to a catalyst for purifying the exhaust gas from a lean burn engine using gasoline as a fuel and a method for the purification.
2. Description of the Related Art
In recent years, the exhaust gases emitting from internal combustion engines such as of automobiles, boilers, industrial plants, and others and containing such harmful components as nitrogen oxides (hereinafter occasionally referred to generally as "NO.sub.x ") have been forming a cause for air pollution. The removal of NO.sub.x from these exhaust gases, therefore, is now being studied from various angles.
In the case of the exhaust gas from an automobile, for example, the method which consists of treating the exhaust gas by the use of a three-way catalyst thereby simultaneously removing NO.sub.x, hydrocarbons (HC) and carbon monoxide (CO) has been heretofore used. This method is implemented under the conditions which are capable of introducing such an amount of air as is necessary for complete combustion of the fuel (the ratio of the air to the fuel hereinafter referred to as "A/F ratio"). When the A/F ratio grows (this situation referred to hereinafter as "state of oxidizing atmosphere" or "lean state"), however, the oxygen is present in an excess of the amount necessary for complete combustion of such unburnt components in the exhaust gas as hydrocarbons and carbon monoxide. In this state of oxidizing atmosphere, it is difficult to effect removal of NO.sub.x by reduction with an ordinary three-way catalyst.
For the removal of nitrogen oxides in diesel engines among other internal combustion engines or in boilers, the method which resorts to use of a reducing agent such as ammonia, hydrogen, or carbon monoxide is prevalent. This method, however, is at a disadvantage in necessitating special devices for the recovery and disposal of the unaltered reducing agent.
The trend of the combustion devices toward higher fuel economy has been gaining in prominence owing to the problem of preservation of resources, the problem of the earth's warming due to the increase of carbon dioxide emitting from internal combustion engines, and others. The measures consequently taken to promote the fuel economy cause the exhaust gases to assume an increasingly lean state and the problem of NO.sub.x reduction to gain all the more in seriousness.
Recently, as a means to remove the NO.sub.x, the methods which reside in using a NO.sub.x -decomposing catalyst formed of a crystalline aluminosilicate containing copper ions have been disclosed (JP-A-60-125,250 and U.S. Pat. No. 4,297,328). These methods, as disclosed in these patent specifications, only go the length of decomposing nitrogen monoxide (NO) into nitrogen (N.sub.2) and oxygen (O.sub.2). They encounter difficulty, however, in permitting effective removal of nitrogen oxides under the actual conditions of the exhaust gas.
The crystalline aluminosilicate generally has no perfect resistance to heat and, therefore, exhibits no perfect durability when it is exposed for a long time to an exhaust gas at a temperature exceeding 700.degree. C.
JP-A-63-100,919 has disclosed a method which consists in treating an exhaust gas with a copper-containing catalyst in an oxidizing atmosphere in the presence of a hydrocarbon thereby preferentially promoting the reaction of the NO.sub.x with the hydrocarbon and obtaining efficient removal of the NO.sub.2. The hydrocarbon to be used for this method, as disclosed, may be either the hydrocarbon originally contained in the exhaust gas or the one to be added to the exhaust gas from an external source. As a specific mode of implementing this method, a procedure which comprises first exposing the exhaust gas to the copper-containing catalyst thereby removing the NO.sub.x and then exposing it to an oxidizing catalyst thereby removing the hydrocarbon, carbon monoxide, and others is also disclosed.
The catalyst mentioned above manifests inferior resistance to heat and, when exposed to an exhaust gas at an elevated temperature, suffers degradation of the NO.sub.x -decomposing property thereof. To cope with these drawbacks, the method which resides in parallelly disposing the catalysts mentioned above and, when the exhaust gas is suffered to reach such an elevated temperature as mentioned above, causing the stream of the exhaust gas under treatment to be bypassed through the oxidizing catalyst or the three-way catalyst has been disclosed (JP-A-01-171,625).
As catalysts for the removal of nitrogen oxides which use precious metals, the catalyst which has platinum, for example, deposited in conjunction with an alkali metal or the like on a carrier of aluminum oxide (JP-A-49-21,379) and the catalyst which comprises activated alumina, platinum, and barium oxide (JP-A-55-67,334) have been disclosed. These catalysts are used under the condition that the exhaust gas under treatment should possess a low oxygen content (state of reducing atmosphere) or should be in the neighborhood of stoichiometry.
As a catalyst which is usable in the state of oxidizing atmosphere, the catalyst which has the components of platinum, rhodium, tungsten, and an alkali metal deposited on a carrier has been disclosed (JP-B-58-45,288). This state of oxidizing atmosphere has 1.4 for its Z number (equivalent to about 14.70 to 14.75 as reduced to the A/F ratio). Thus, this catalyst is substantially fit for use in the neighborhood of stoichiometry.
As catalysts which are usable in the state of oxidizing atmosphere, various catalysts using platinum as a main active component thereof have been disclosed (WO93/12863, WO93/07363, WO93/08383, JP-A-04-367,724, JP-A-05-168,860, JP-A-05-187,229, JP-A-05-261,287, JP-A-05-317,652, and JP-A-06-31,139). While the catalysts using platinum excel in terms of performance, the use of platinum proves unfavorable because platinum is so expensive as to boost the production cost of the catalysts. Besides, platinum offers no high resistance to heat and possibly proves imperfect from the viewpoint of durability. It is, therefore, necessary that the amount of platinum to be used in a catalyst be decreased or the use of platinum in a catalyst be avoided.
An object of this invention, therefore, is to provide a purifying catalyst for the removal of nitrogen oxides from an exhaust gas emitting from a lean burn engine using gasoline as a fuel and a method for the purification of the exhaust gas.